Abstract:
Animal Production in Australia Vol. 15 EFFECT OF RESTRICTED GRAZING IN EARLY LACTATION ON MILK PRODUCTION FROM DAIRY HEIFERS J. A. STEWART* and J. W. TAYLOR* SUMMARY Dairy heifers had an estimated grazed pasture intake of either 11.8 (H) or 6.0 (L) kg DM/heifer/day for four weeks in early lactation and a common pasture allowance thereafter. Group L produced less milk (2202 vs. 2433 R) and milk protein (75.0 vs. 81.6 kg) than Group H in 31 weeks but milk fat yield (104.9 vs. 109.8 kg) and milk composition were not significantly affected. All heifers lost body condition over 31 weeks but nevertheless gained live weight. INTRODUCTION In Victoria, most cows and heifers calve in late winter and there is often a period in early lactation in which there is insufficient pasture available to fully feed stock. Several recent grazing experiments (Bryant and Trigg lY79; Grainger and Wilhelms 1979; Bryant 1980; Glassey et al. 1980) have shown the production penalties incurred by underfeeding mature cows in early lactation. These are related to the severity, duration and timing of underfeeding but the immediate effects are relatively small when compared to increases in pasture utilization and consequent gains in per hectare production (Bryant 1980). These experiments have also shown that residual effects for the remainder of the lactation are no more than the immediate effects if cows are subsequently fed to appetite. Similar information for heifers grazing pasture is heifers are still growing when they calve at two years of underfeeding in early lactation may differ from those in investigated the effect on milk yield and composition of grazed pasture for a four week period in early lactation. MATERIALS and METHODS The experiment was conducted at the Dairy Research Institute, Ellinbank near Warragul, Vic. on pasture consisting predominately of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens). Animal management and treatments Forty-two Jersey, Friesian x Jersey and Friesian two-year-old heifers were grazed on sufficient pasture to maintain their body condition score (Earle 1976) over the last two months of gestation. Mean calving date was July 22 (+I1 days). From calving until August 12 all heifers were grazed together on a liberal allowance of pasture. They were then allocated at random to two groups and offered either pasture ad libitum (Group H) or a restricted daily allowance of pasture (Group L). At this time their mean live weight was 323 (242) kg with a mean condition score of 4.9 (+0.2). unavailable. As most age the effects of mature cows. This study underfeeding heifers on * Department of Agriculture, Dairy Research Institute, Ellinbank, Vic. 3820, 608 Animal Production in Australia Vol. 15 TABLE 1 Means and standard errors of pasture allowance and estimated intake (kg DM/heifer/day) and residual pasture after grazing (kg DM/ha) TABLE 2 Effect of feed restriction in early lactation on milk yield and composition and on condition score and liveweight change -f- Values are covariace - adjusted means * Indicates a significant effect of treatment 609 Animal Production in Australia Vol. I5 Depending on pasture availability, Group H was offered a fresh paddock each 24 or 48 hours to provide sufficient pasture to satisfy appetite. This was based on minimum residual pasture yields of about 1500 kg DM/ha, anticipated to provide daily intakes of about 11 kg DM/heifer. Group L was grazed on an adjacent area for four weeks. Their daily allowance of pasture, anticipated to provide an intake of 7 kg DM/heifer/day, was controlled with electric fences. Pasture allowances and estimated intakes achieved in the experiment are presented in Table 1. After four weeks of different feeding both groups were grazed together on pasture which was judged to satisfy their requirements until about mid November. Thereafter, declining pasture quality and, by late summer, quantity of available pasture would have limited intake. Heifers were dried off when their weekly milk production fell below 21 R or on March 24 whichever was earlier. Measurements Milk yield was recorded twice daily and a composite sample of p.m. and a.m. milk, taken once weekly, was analysed for fat and protein content. Live weights and condition scores were recorded weekly until mid November, then fortnightly. Linear regressions of live weight and condition score with time, were calculated for individual animals for the periods O-4, 5-10 and 24-32 weeks. These were then used to predict individual live weights and condition scores at 0, 4, 10 and 31 weeks. During the experimental period and the following three weeks, pasture present before and after grazing was measured daily using a rising plate pasture meter (Earle and McGowan 1979). From these measures, group mean daily pasture allowance and intake were calculated. Statistical analyses Data were analysed by stepwise regression significance but with a treatment pseudovariate were production measures for the week preceding weight at the start of the experimental period. date and treatment x covariate interactions were they significant (P>O.O5). RESULTS Persistency of lactation was similar for both groups as measured by the number of heifers drying off before March 24 (5 in each group) and the weekly level of milk production on March 24 (H, 29 R vs. L, 31 R; P>O.O5). Milk production, condition score and live weight results are presented in Table 2. There was an immediate effect of underfeeding on all measures except liveweight change and, apart from milk fat content, these effects were evident for a further six weeks. Total yields of milk and milk protein were lower for Group L but other production measures were not significantly different. Total liveweight gain was greater for Group L but both groups lost a similar amount of condition score. operating at the 5% level of forced in. Covariates included the experimental period and live Initial condition score, calving also tested but in no case were 610 Animal Production in Australia Vol. 15 DISCUSSION Despite the difference of 5.8 kg DM/heifer/day in estimated feed intake for four weeks, differences in production were small. It is considered unlikely that the effect would have been greater if underfeeding had been imposed on all Group L animals from calving since our analysis did not indicate any effect of time of calving. Furthermore Grainger and Wilhelms (1979) reported similar responses to underfeeding with pasture in either the first or second five weeks of lactation. The method adopted in this experiment is representative of farm practice, where a period of underfeeding is likely to start at a different stage of lactation for each animal. In terms of milk fat production, the immediate effect of underfeeding represents a marginal efficiency of 116 kg pasture DM intake per kg milk fat, in contrast to recently reported figures for mature cows which range from 19 to 45 kg pasture DM per kg milk fat (Bryant and Trigg 1979; Grainger and Wilhelms 1979; Bryant 1980; Glassey et al. 1980). This suggests that the immediate effect on the production of heir'ers, of underfeeding in early lactation, is less severe than for mature cows and warrants further investigations. Although the lactation difference of 4.9 kg milk fat was not statistically significant it would represent, in economic terms, a marginal efficiency of 33 kg pasture DM per kg milk fat and a residual effect of about 3.5 times the immediate effect of underfeeding. This apparent residual effect is greater than that recently reported for mature cows on pasture (Bryant and Trigg 1979; Grainger and Wilhelms 1979; Glassey et al. 1980). However, as discussed by Grainger and Wilhelms (1979), there has been considerable variation in earlier reported residual effects and, since these have a considerable influence on economic consequences, there is a need to more clearly define the factors responsible for the variation of residual effect. In marked contrast to mature cows (Glassey et al. 1980) heifers gained live weight while they were losing condition score. This is presumably a reflection of the propensity of heifers to continue growing towards mature size. Heifers underfed in early lactation gained relatively more live weight in later lactation as had been reported for cows (Grainger and Wilhelms 1979). The implication is that heifers tend to partition nutrients between growth and milk production and support both by mobilizing body fat. This suggests that heifers should be well grown and have a high body condition score at calving. ACKNOWLEDGEMENTS We would like to thank the following people : Miss B. Cooke (technical assistant), Miss D. McInnes and Mr I. Scott (milking and care of stock) and Mrs. J. Pedder (milk analyses). Miss Cooke's salary was provided by the Australian Dairy Research Committee. REFERENCES BRYANT BRYANT EARLE, EARLE, GLASSE , A.M. (1980). Proc. N.Z. Soc. Anim. Prod. 40:5O. 39:139. , A.M., and TRIG-. (1979). Pra.2. S8?. Anim. Prod. = D.F. (1976). J. Agric. Vic. 74:228. D.F. and MC GOWAN, A.A. (w@).=Aust. J. Exp. Agric._Anim. Hush. & 337, Y, C.B., DAVEY, A.W.F., and HOLMES? C.W. (1980). Proc. N.Z. SOC. Anim prod, &:59. 19:395. GRAINGER, C., and WILHELMS, G. (1979). Aust. J. Exp. Agric. Anim. Hush. p9 . . 611